Gear, Parts and Accessories

CMMG’s Lower Parts Kit

CMMG box next to a lower parts kit

Written by Be a Blogger for a Day Contest Winner, Michael Petty.

I was sitting at my desk when I received a text message. It was my wife alerting me to the arrival of a package. I love this sort of text, and who wouldn’t? Especially when we’re talking about parts for an AR build! I drove home that day anxiously wanting to get my hands on the new arrival. I arrived, walked inside and proceeded to walk around the table at least a dozen times where the box sat. I like to let the anticipation build and build and it did. I walked around a few more times and couldn’t keep myself from it any longer.

Once inside the box, I have to admit, I was impressed with the packaging. It wasn’t the standard brown cardboard and package materials that impressed me though, it was the treasure inside. The CMMG box containing all the goodies was a work of art! Covering the surface was a CAD-like drawing of the lower parts kit (LPK) as they would be assembled in the receiver. I thought. “Who needs YouTube or fancy schematics with a box like this?” I studied the detailed drawing trying to imagine the parts working together performing their respective tasks. I was beginning to think highly of CMMG. I wondered what else it had in store for me. I hit the CMMG web page to learn more.

I liked what I discovered about the company making these nice LPKs. The company has been in the firearm business going on 12 years, manufacturing both complete rifles as well as parts and parts kits such as the mentioned LPK. They are a God-fearing group of people, which I can certainly admire, and they aren’t afraid to stand behind their products with a lifetime warranty against defects and workmanship. You can do this sort of thing when you do things right. All of their parts are made in the USA. I could hardly wait to see if the parts lived up to CMMG’s claims. I can report that I was not disappointed.

The pistol grip lay loose in the box and other pieces were categorized and contained in color-coded baggies. A detailed information card was included listing all the parts by name and color category. I was ready to open some baggies! This parts kit went in a polymer lower I had been waiting to build up. I’ll admit, first I held the pistol grip in place and aimed my soon-to-be AR out the window into to a field where I imagined coyotes or at least steel targets ready to ring out.

Snapping back to reality, I grabbed the trigger and test fitted it. Like a glove, it fit right into the trigger slot and a quick sight test showed the pinhole was properly aligned. This was the norm for all the parts, from the grip to the take down pins and springs. Everything seemed to be perfectly matched for all the little slots, holes and channels they would call home.

Once I got over the play-and-pretend period, I sat down for the task of assembling the lower. Within 10 minutes, maybe 15, the LPK was assembled. I pulled the hammer back, and the sear caught and held. Holding the hammer and pulling the trigger allowed the hammer to drop slowly without any damage to the lower or other pieces in the kit. Keeping the trigger held to simulate an actual firing cycle, I pulled the hammer back once more and this time the disconnect caught. Perfect! Releasing the trigger allowed the disconnect to let go of the hammer and once more become secured by the sear. Having observed proper operation of parts, I felt even more excitement knowing this was one-step closer to achieving my AR goals.

At this point I was waiting on my upper to arrive. Once in hand for the build and test fit, I would then be ready for a trip to the range. In the meantime, I decided to make a phone call. A good friend had completed several AR builds in different calibers. The phone rang and Gary picked up, “Hey man! Guess what I have?” “No idea,” he replied. I paused for dramatic effect. “I got an AR lower in need of an upper! Can you help a brother out?” I asked. I loved what came next, “What ya want, .223, 9mm, .300…?” The list went on. I responded with, “Let’s just go with .223.” A smart choice as I had a small surplus of ammo in that caliber.

I met Gary the next day at his house; we attached the upper, loaded a mag and let the fun begin! Gary let me go first since it was my ammo, my lower and my newly installed CMMG LPK. I’m still not sure if it was out of respect or for his own safety. I’ve often been ribbed a little about doing all my own work and whether it’s safe or not.

I reminded myself of the quality parts I had chosen, charged the rifle, pulled the stock close to my shoulder, looked through the optic and pulled the trigger. I sent the first .223 round out to the side and down range about 75 yards through a paper target and into a dirt hill backstop. “Whew! That was all right, nothing exploded!” I said with relief. I settled down and fired about 10 more shots at a rapid pace, taking only 2.5 seconds. It wasn’t long before Gary wanted a go at it. He too was impressed with the lower as I reminded him of my quality work, the quality parts, and the deal I had found on this little kit.

The trigger was solid with no creep, just like most other single-stage AR triggers I have pulled. The trigger is of the generic type, but is plenty comfortable on my trigger finger even when firing quick consecutive rounds. I could not detect any burrs, sharp edges or anything to hinder the joy of shooting this would-be AR.

The grip felt nice too, giving enough real estate to know you were holding something. You will never have to wonder what you’re holding with this thing in your hand. The magazines all went in smoothly, held securely and released easily after pressing the magazine release button. The take down pins were easily set and released to attach or remove the upper. From every little spring to the feel of the trigger itself, I am excited to have this kit as a part of my AR build. I recommend it to anyone looking for a good quality LPK for a custom AR build. Now I wait anxiously for the next text message!

Michael M. Petty lives in North Carolina. He has been married for 20 years and is the father to five children. Even though he works a lot, he still finds time to train and play. His home defense gun of choice is a Mossberg 500 12-gauge shotgun—more effective than a baseball bat! For carry, he chooses a SCCY CPX-2. Currently in the process of building his first AR-15, Michael can hardly wait for it to be complete. He says, “I always said I wanted a shotgun, handgun and then a rifle, things are going along quite nicely. I look forward to sharing in the firearms tradition and continuing it with my children.”

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (15)

  1. I have one thing to add to Michael’s experience that is one of mine. The CMMG has a beautiful web video that takes a builder through the lower installation step by step. It’s perfect! A couple of years ago I was putting together my first build and had some trouble following instructions with a (shame on me) other than CMMG LPK. I went on the web and found their video and was able to assemble the complete lower in 15 minutes. The beauty of the video is that you can sit in front of your computer monitor, lay out the parts as shown in the video and then pause at each stage to get a good look at what you are doing (or are about to do). The video convinced me of the quality of CMMG support which also convinced me to buy their LPKs. Like Michael, I have not been disappointed with any one of them, I have put in no less than 5.

  2. Overbuilt many AR’s from 80% lowers. All calibers. I just used the CMMG lpk on the last two i built. I have to say, the safety felt like crap. It was gritty and had no positive detention snap. I thought maybe the lowers were out of spec so I tried some extra selectors I had left over from ambi conversions. The others were smooth and snappy. So aside from the terrible safety selector and detent the parts kits are decent. I wouldn’t say great or anything special. The FCG needs lots of polishing with 800 grit sandpaper to get rid of tool marks on contact points for a slightly decent trigger. Replace the springs with the JP $10 enhanced FCG spring kit and it will bring down pull weight to around 4lbs.

  3. Daniel you have confused me even more! Following your suggestions, went to the 80% site—- they show 9 options! Which one did you pick to build for the 6.5mm Grindel ?
    I have not looked for an A2 stock, but I think I would like an adjustable one.
    What one would you suggest?
    On the magazines, since this is for the 6.5mm Grindel, I suppose Alexander would be the best bet, yet I have seen other offerings. What did you chose?
    What is an “Ergo” grip ?
    Magazine Release button? Isn’t that in one of the kits?
    I am in Florida, not CA — the land of fruits and nuts — two legged variety among some!
    Please give me some more details for this very ignorant old boob !

    1. For 80% rifle lowers they make an AR-15 and an AR-10 size. You want the AR-15. Which AR-15 you choose is up to you. Like I said, I think you will be happier with a purchased lower. Look for the videos on machining a lower to see if it is something you want to do.

      I don’t have recommendations on collapsable stocks because I do not use them.

      Ergo Grips is company name.

      If you live in Florida, do not worry about a California mag release button.


  4. Has anyone tried to use a CMMG LPK on a Polymer80 80% lower? I purchased their 80% .308 lower and am looking for a well-built, compatible (DPMS-style) LPK.

    1. I used the CMMG LPK on a Polymer 80 lower to build a 300BLK pistol.
      Works great. Just added a laser and have not yet tried that out.

  5. I am an old poop from the M-1 days. I cannot shoot high-power rifles due to an old back injury, so I disposed of those guns years ago I have no knowledge of the AR systems, but my interest was tweaked when I saw an article about the Alexander 6mm Grindel and its very low recoil energy.
    Now, would this CMMG lower work with an Alexander upper in this 6mm Grendel caliber ?
    I like the idea of a single stage trigger.
    Can anybody comment on this combination ?

    1. Yes, you can. I built an 80% lower for an AR-15 with an Alexander Arms 6.5 Grendel upper attached to it and I used the CMMG parts kit. I have never had a problem with it and it is my Antelope gun, where rifles are permitted.

      I bought the compensator from Alexander Arms, which you may want to do with your back problems. I have shot it with and without the compensator and the recoil is not bad in either case, but definitely less with the compensator on.

      The hardest part of the assembly was getting the roll pin for the bolt catch in: on one of my lowers I had to go buy a new roll pin.

      Personally I like the silky smooth, clean break provided by a single-stage Timney or Velocity trigger. I use both.

  6. I built my first lower with the CMMG parts kit. Went together flawlessly and I have shot at least 500 rounds with no problems.

  7. I know this article is about CMMG’s LPK, but I want to put a plug in for their dedicated .22 LR “M4” upper receiver. What a great product! Mine is sub-half-MoA out beyond 50 yards, reliable, light, fast, & about as perfect as a dedicated .22 LR AR upper can be.

    It uses Black Dog magazines – unlike the S&W (& others) that have proprietary mags (stupid, in my book). The BD mags also fit my Match .22 LR “Spacegun” which is built around the CMMG conversion kit, and the Accuracy Speaks matching barrel.

    CMMG is innovative, believes in quality & reliability, and keeps costs affordable. They are one of my “go-to” companies.

    Now if the “food” for these rifles would become more available, I would be able to shoot a lot more!

  8. I use the CMMG LPK on all of my 80% lowers. The kit has everything I need without including several things I do not need (and do not need to pay for).

    Note: always wear eye protection when shooting. Especially if you are taking photos for publication.

    1. Thanks for your comment Daniel. Not being at all familiar with an AR build, could you please send me a list of all the pieces/parts that you used to build your 6.5mmGrendel rifle. Name the brands if that applies please.
      Based strictly on concept, I think I would want an aluminum lower unit.
      Did you buy a kit and complete one, or did you use a serially numbered commercial one? What sights do you have on it?
      Thanks for a comeback.

    2. With the AA upper and this parts kit you are a long way there.

      You need a lower. I built mine from 80 percent arms, partially because I live in California and partially because I just wanted to make it my own, but if you live in a state that is easy to buy a lower, you probably want to buy the lower.

      And you need a stock and a grip. I never wear body armor so I stick to an A2 stock, but a lot of people like a collapsable stock.

      I am a fan of the Ergo grip, but there are a lot of them out there.

      That’s really about it. If you haven’t built an AR-15 before, I suggest finding someone who will do the first with you because it will go a lot easier. The detents can be a pain until you have done it once.

      Oh, and you need a magazine, and if you are in California, you need a California magazine release button.

      I think you will find it is very rewarding to build your own AR-15. I have three now and am planning my fourth.


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